Concrete slab floors

Concrete slab floors come in many forms and can be used to provide thermal comfort and lifestyle advantages. Concrete slab floors provide good thermal mass, which works particularly well in cool climates and climates with a high day–night temperature range. Thermal mass needs to be combined with other passive design principles to be effective.

Concrete has high embodied energy, but this can be offset by its permanence. If reinforcement is correctly designed and placed, and if the concrete is placed and compacted well so there are no voids or porous areas, concrete slabs can have an almost unlimited lifespan.

Types of concrete slab floors

Some types of concrete slabs may be more suitable to a particular site and climate zone than others.

Slab-on-ground

Slab-on-ground is the most common type of slab. There are 2 types: conventional slabs with deep excavated beams and waffle pod slabs, which sit near ground level and have a grid of expanded polystyrene foam pods as void formers creating a maze of beams in between. Conventional slabs can be insulated beneath the broad floor panels; waffle pods are insulated beneath. Both may benefit from slab edge insulation.

Suspended slab

Suspended slabs are formed and poured in situ, with either removable or ‘lost’ nonloadbearing formwork, or permanent formwork that forms part of the reinforcement.

Precast slab

Precast slabs are manufactured off site and craned into place, either in finished form or with an additional thin pour of concrete over the top. They can be made from conventional or post-tensioned reinforced concrete, or from autoclaved aerated concrete (AAC).

Photo: Envirotecture

Photo: Envirotecture

Photo: Envirotecture

Photo: Austral Precast

Environmental impact of concrete slab floors

Conventional concrete has high embodied energy and embodied carbon, but there are ways to reduce the environmental impact of concrete.

Concrete has 3 main components: coarse aggregate (stone), fine aggregate (sand) and cement, with water added to the mix to catalyse the reaction that causes it to solidify. Concrete’s main environmental impacts are greenhouse gas emissions from cement production, and the mining of raw materials.

Using cement extender

Portland cement is the most common type of cement used in concrete. Replacing a proportion of the Portland cement with waste products called ‘extenders’ can significantly reduce embodied energy and greenhouse gas emissions. These extenders (also known as ‘supplementary cementitious materials’) are commonly available from most concrete batching plants.

Various blended cements are available, some with high proportions of extenders (up to 85%) replacing Portland cement. Extenders include fly ash, ground blast furnace slag and silica fume which are waste materials from other manufacturing processes. New technologies include the use of reactive magnesia in combination with Portland cement.

Using recycled aggregate

Coarse aggregate and sand can be replaced by recycled materials such as crushed concrete from demolition, slag aggregates and recycled sand. Such replacement decreases the amount of material going to landfill, reduces embodied energy, and can lower costs. A common approach is to use 30% recycled aggregate for typical structural concrete. There is no noticeable difference in workability and strength, although a structural engineer should always specify the final mix. It is possible to use up to 100% recycled coarse aggregate in concrete under controlled conditions.

Recycling of masonry can also produce finely ground sand, as can other industrial by-products such as ground glass, fly ash, bottom ash and slag sands. However, the properties of these products can change the characteristics of concrete, and should always be used with expert engineering guidance.

While slag aggregates are readily available in areas close to steelworks, cartage costs may prohibit their use in more remote areas. For similar reasons, manufactured sands and crushed concrete may not be readily available in all areas.

Keep recycling in mind when demolishing concrete on site. Concrete can be crushed for reuse in new concrete. Storing it separately from other demolition materials will help to achieve a more usable product. Recycling concrete is cost effective, minimises waste, and reduces the need to use more of earth’s natural resources.

Recent innovations

New cements such as geopolymers (‘e-concrete’) and magnesium cements reduce greenhouse gas emissions. Some of these new cements set by absorbing carbon dioxide, which dramatically reduces the carbon footprint of the concrete.

Alternative forms of concrete with very low embodied energy are also becoming more widely available, such as hempcrete. This uses industrial hemp fibres in conjunction with lime-based binders to sequester carbon dioxide for the life of the building. Hempcrete is based on an ancient technology and uses modern production techniques to make it cost effective.

Eco-comparison websites can help you select options with low lifecycle environmental impact (refer to References and additional reading).

Site considerations

Reactive soil sites

Sites with reactive soils can be difficult to build on, but ‘floating’ stiffened concrete raft slabs cope well with these conditions. Some stiffened raft slabs (known as waffle raft slabs) use void formers at regular intervals, forming closely spaced, deep reinforced beams criss-crossing the slab underside.

Void formers are expanded foam boxes that insulate the slab, but ground-coupled alternatives are available. These include proprietary systems that use recycled tyres or reused detergent bottles filled with water, grouped together as void formers.

Steep sites

Steep sites may make slab-on-ground construction impractical. A suspended slab may be a suitable way to gain the advantage of thermal mass on a steep site. Typical pole frame construction can be adapted easily to incorporate a slab. The slab underside should be insulated in most climates.

Permanent structural formwork or one of the many precast flooring alternatives is usually the most cost-effective way of constructing elevated suspended slabs. Precast flooring alternatives are normally designed by an engineer and installed by builders or specialist subcontractors. Suspended concrete floors can be supported on timber, masonry or steel structures, and allow fast installation on site. However, their loads are higher and structural support at each end must be given more attention.

High winds

In cyclonic areas, concrete slabs, especially on ground, are a means of anchoring the whole building against extreme wind loads. The structure must be engineered holistically to ensure compliance with the relevant building codes.

Design considerations

Passive design

Passive design principles and high-mass construction work well together in many climates, and concrete slabs are generally the easiest way to add thermal mass to a house. Living rooms should face north, in all but hot humid climates, to enable winter sun to invest warmth into the slab. Concrete slabs perform better as the diurnal temperature range increases.

Natural ventilation should be provided for in the design, to allow heat stored in the slab to dissipate on summer evenings, particularly if there are slabs on upper storeys, where warm air accumulates. Zone off the upper space from lower living areas where possible and ensure the space can be naturally ventilated. This is particularly important if bedrooms are located upstairs, to maintain night-time sleeping comfort.

Balconies that extend from the main slab of a house act as a thermal bridge, conducting uncontrolled heat into or out of the building. Design these slabs to be thermally independent of the main slab by incorporating an insulator at the joint, concealed beneath the external doors and walls.

Slab insulation

A slab-on-ground can be ground coupled (uninsulated) or insulated. An uninsulated slab in a good passively designed house has a surface temperature approximately the same as the stable ground temperature at about 3m depth. Depending upon your location, this may or may not be desirable.

Ground coupling in mild climate zones such as Perth, Brisbane or coastal New South Wales allows the floor slab of a well-insulated house to achieve the stable temperature of the earth: cooler in summer, warmer in winter. In winter, added solar gain boosts the surface temperature of the slab to a comfortable level. In northern Australia, ground coupling works well unless the building is to be air-conditioned, in which case insulating the slab — especially the edges — is essential.

In climates with colder winters, such as Melbourne or the Southern Highlands of New South Wales, the deep ground temperature is too low to allow passive solar heating to be effective enough. In these locations, it is recommended that slabs are insulated underneath (to decouple the slab from the ground), which reduces the amount of heat required to achieve comfortable indoor temperatures.

Slab insulation can be done with large sheets of high-density foam, laid all the way under the floor panels, with only the edge and internal beams penetrating to foundation level. Alternatively, the common and cost-effective waffle pod slab supplies sufficient insulation in all but alpine climate zones.

Waffle pods are interconnected dense closed cell foam. Preferred foams are extruded polystyrene (XPS) and polyisocyanurate (PIR) foams, which also have higher R values. Expanded polystyrene (EPS) foam breaks up easily on site, causing damaging pollution, and is not recommended. Pods can also be made from old car tyres filled with compacted fill. This system maintains earth coupling whereas cardboard and foam systems do not. Alternatively, arrays of linked recycled plastic domes may be used (which reduces the amount of concrete needed).

Photo: Light House Architecture & Science

Insulating the edges of floor slabs prevents warmth escaping through the edges of the slab and is beneficial in all but the mildest climates. It is possible to retro-fit slab edge insulation to existing slabs-on-ground. Although renovations are an ideal time to do this, it can be done at any time. Seek advice from an engineer on disturbance to foundations and reinstatement of material, and do not breach termite barriers. Edge insulation can increase termite protection, by using termite barriers (poly sheets embedded with termiticide) as the slab’s vapour barrier membrane.

Acoustics

Generally, concrete slabs are a good way to reduce the transfer of music or conversation noise from one level of a home to another, and between rooms on the same level. These airborne noises are not transmitted through a slab, but impact noises are, for example, high heels on a tiled floor can be heard in the room below. An acoustic barrier can be included in the ceiling to reduce such noise.

Open-plan houses may transmit more noise from one living area to another than is convenient. Thermally efficient hard flooring exacerbates this, so other elements in the room need to be designed to limit noise.

• Design the floor plan to be able to close spaces off from each other when needed.
• Large flat ceilings can reflect too much noise. Dropped bulkheads, sloped ceilings or suspended cupboards around kitchens help to absorb and dissipate sound, especially if lined with textured or softer materials.
• Use absorbent materials on wall panels or add large fabric wall hangings. Heavy drapes and curtains can also help to absorb sound.
• Autoclaved aerated concrete (AAC) floor panels have around 30% of the mass of normal concrete and therefore offer significant acoustic benefits along with thermal comfort due to their insulation properties.

In-slab heating

Additional heating may be required if there is inadequate solar access to the concrete slab. Because concrete slabs offer so much thermal mass, they lend themselves well to long-cycle in-slab heating systems, provided they are efficient. In-slab heating’s slow response time of 2 hours or more makes it unsuitable for part-time occupancy or for sites where it may be required intermittently, unless it is purely solar powered. Slab heating is best suited to houses with permanent or high occupancy, where it is in operation for the whole of winter. Insulation is required in all cases to reduce heat loss to the ground.

Photo: Envirotecture

Slab heating may be:

• Solar boosted hydronic heating – Hydronic heating is very energy-efficient: solar energy is picked up by rooftop collectors, and water (which has very high thermal mass) is used to carry it into the slab via pipework. It is especially useful where windows and doors are not exposed to the sun. When designed properly, hydronic heating has very low running costs. Warm water delivers heat to the slab through pipework embedded near the top of the concrete. Solar hydronics can also be boosted by a range of energy sources, including ground-source heat pumps, gas burners, and heat recovery units. Unlike electric coil heating, in low-humidity climate zones, hydronic heating can be reverse cycled in summer, dumping excess heat into the night sky.
• Electric resistance heating – Electric resistance heating coils have been the most common type of slab heating and are attached to the reinforcement before pouring the slab. The heating coils are usually controlled by timed switching so that a relatively even temperature can be maintained over a daily cycle with top-up periods of just a few hours a day. Unless such a system is powered by renewable energy, such as a large rooftop PV system, it will incur high running costs and greenhouse emissions.

Finishing

For the thermal mass of a concrete slab to work effectively, it must be able to interact with the house interior. Covering the slab with finishes that insulate, such as carpet, reduces the effectiveness of the thermal mass. Look for solutions that expose the thermal mass, such as tiles or concrete finishes.

Tiles fixed by cement or cement-based adhesives are commonly available in many colours, sizes and patterns, but avoid rubber-based adhesives which have an insulating effect. Darker colours with a matt surface work better than light shiny finishes. Choices include ceramic tiles, slate tiles, terracotta tiles, pavers and bricks.

Polished concrete offers a variety of finishes: trowel finished floors (with or without post-applied finishes), and ground and polished or abrasive blasted floors. Many finishes can be used in combination to achieve a wider range of results, to suit any style or taste. Trowel finishes include:

• steel trowel finish, where a normal hand or machine trowelled finish is used for the surface of the slab, usually with a clear sealer applied, preferably a low volatile organic compound sealer
• burnished concrete, where the surface is finely steel trowelled, bringing the surface up to a glossy finish free of any trowelling marks.

Coloured concrete can be used in either steel trowel or burnished finishes to achieve various results. It is advisable to use experienced specialist contractors to carry out this work. The colours can be applied as oxides in the mix, or as ‘dry shake’ pigments applied to freshly screeded concrete and then trowelled in, or as chemical staining.

Chemical stains are used with either steel trowel or burnished finishes. Metallic salts are carried into the surface of the concrete by mild acids, making the stains deep and permanent. Saw cuts can be added to enhance or separate panels of colour.

Ground and polished finishes include:

• exposed aggregate, where the normal grey concrete is ground back by several millimetres to expose whatever aggregate exists in the slab; often used in renovations of older buildings to reveal some of their history
• exposed selected aggregates, where the cement colour and aggregate in a new slab are carefully selected so that when the surface is ground back they produce desired effects
• abrasive blasting of the concrete surface to reveal varied effects and give a safer surface particularly in areas that may be prone to moisture, including entrances and wet areas.

Photo: Ben Wrigley (© Light House Architecture and Science)

Toppings such as terrazzo can also be used on their own or together with some of the effects listed above to provide interesting visual finishes that do not interfere with thermal performance.

Some options require careful protection of the slab during subsequent construction works.

Renovations

Renovations can often incorporate concrete slabs, even when the original building does not. Added rooms can use slab-on-ground or suspended slabs. When renovating rooms with timber floors, it is often possible to replace the timber with a concrete slab for added thermal mass and quietness underfoot.

These slabs can be suspended on the original subfloor walls and footings, or if the old floor is close to the ground you can choose an infill slab on fill. Most advantage is gained if passive design principles are followed.

Termite protection to both the new and old structures requires careful attention at the joint between them. Take care to construct continuous physical barriers, and always provide full inspection access to the junction in houses with raised timber floors.

Construction considerations

Cracking and durability

To ensure longevity and structural integrity of the slab, control cracking with:

• proper preparation of foundations
• appropriate water content; excess water causes cracking and weakens the slab
• appropriate placing and compaction. Compaction during placement is usually achieved by vibrating the concrete. This reduces the air entrapped in the concrete, giving a denser, stronger and more durable concrete better able to resist shrinkage cracking. Deeper beams should be compacted; thin slabs (typically 100mm thick) receive adequate compaction through the placing, screeding, and finishing operations.
• appropriate curing; employing a curing membrane in the first 3–7 days (continuous wetting is a common practice, but also consumes large amounts of water)
• appropriate construction scheduling allowing 28 days, or the duration specified by your structural engineer, for the concrete to reach design strength before placing significant loads.

Termite protection

For minimum termite risk, concrete slabs should be designed and constructed in accordance with Australian Standards to have minimal shrinkage cracking. Joints, penetrations and the edge of the slab should be treated:

• Slab edge treatment can be achieved simply by exposing a minimum 100mm of slab edge above the ground or pavers, forming an inspection zone at ground level.
• Where a brick cavity extends below ground, physical barriers must be installed using sheet materials including stainless steel, a termiticide-impregnated polyethylene vapour barrier and/or damp course, a fine stainless steel mesh, or finely graded stone.
• Pipe penetrations through concrete slabs require a physical barrier. Options include sheet materials such as termiticide-impregnated polyethylene vapour barrier, stainless steel mesh or graded stone.
• Although physical barriers are environmentally preferable, chemical deterrents are also available, which must be reapplied at regular intervals to maintain efficacy. Benign natural deterrents can be applied by permanent reticulation pipework similar to a drip irrigation system.

• Eco-comparison websites
  –    Australian National Life Cycle Inventory Database
  –    Building Products Information Rating
  –    Ecospecifier Global
  –    Environmental Product Declaration Australasia
  –    Global GreenTag
  –    Good Environmental Choice Australia
• Ash Development Association of Australia.
• Australasian (Iron and Steel) Slag Association
• Baggs S, Baggs J and Baggs D (1991). Australian earth-covered building. New South Wales University Press, Sydney.
• Cement Concrete and Aggregates Australia.
• Cement Concrete and Aggregates Australia (2016). Building better homes with concrete [PDF].
• Cement Industry Federation.
• Clarke D (2016). When is a concrete floor the right choice? Sanctuary Magazine, Melbourne, Victoria.

Learn more

• Read Thermal mass to understand how to make your concrete slab floor work effectively in your climate
• Explore Passive heating and Passive cooling for tips on designing your home to work well in winter and summer
• Look at Precast concrete to learn how you may be able to save time with this method of construction

Authors

Original author: Dick Clarke 2013

Contributing authors: Bernard Hockings, Caitlin McGee 2013

Updated: Department of Industry, Science, Energy and Resources 2020